1. Field of the Invention
The invention relates to a multistage speed reducer for high speed rotation and, more particularly, to a multistage speed reducer using a planetary roller friction reducer for the first stage reducer and a geared reducer for the second stage reducer.
2. Description of the Prior Art
In speed reducers for transmitting high speed rotation at a high reduction ratio, generally a combination of multistage reducers has been employed for sequentially reducing the rotation speed, resulting in a space consuming mechanism. Geared reducers are satisfactory in applications for high torque rotation, but it has been found that their usefulness is limited in applications for high speed rotation and they create noise problems. In the arrangement of planetary roller friction reducers in two stages, the second stage reducer necessarily becomes very extensive. Although in planetary roller friction reducers, a normal preload is applied to the rolling surface in accordance with the torque to be transmitted in order to prevent the occurrence of gross slip, an excessive torque will cause gross slip on the rolling surfaces, resulting in heat and seizure generation. This problem becomes serious particularly where a high speed rotation is applied to the input.